The kraft process for pulping is the dominant method for producing pulp and paper. In the kraft process, white liquor containing the active cooking chemicals, sodium hydroxide (NaOH) and sodium sulfide (Na2S), are used to cook the wood chips to separate cellulose fibers from lignin. Spent cooking chemicals and lignin are then washed away from the cellulose fibers with water, thus forming a residual spent pulping liquor stream called black liquor. Black liquor, which may initially contain up to about 20% dissolved solids (sometimes referred to as “weak” black liquor), may be concentrated in a series of multiple-effect evaporators and concentrators, for example, up to approximately 75% solids. This concentrated black liquor may then be burned, for example, in a chemical recovery furnace or boiler to recover the fuel value of the black liquor as steam, along with the inorganic chemicals as a “smelt” of sodium carbonate (Na2CO3) and sodium sulfide. (The concentrated black liquor may also be heated with steam to lower its viscosity prior to combustion in the chemical recovery furnace or boiler.) The “smelt” may then be dissolved in water to form green liquor, which may then be reacted with quick lime (CaO) to convert the sodium carbonate into sodium hydroxide to provide effective alkali (−OH), and to thus regenerate the original white liquor.
Because black liquor contains sodium sulfide and other sulfur compounds which are malodorous, or which may form hydrogen sulfide and/or other gaseous malodorous sulfur compounds, black liquor may be subjected to oxidation prior to being burned in a chemical recovery furnace or boiler. This oxidation procedure is commonly known as black liquor oxidation or BLOX. BLOX may provide the ability to control and reduce sulfur emissions from the kraft process. The purpose of BLOX is to reduce the residual sulfide content in black liquor by oxidizing and converting the sulfides, such as sodium sulfide and/or other less stable sulfur compounds, to more stable sulfur compounds, for example, sulfates and thiosulfates, prior to contact with hot flue gases in the chemical recovery furnace or boiler. In North America, about one-third of the paper pulp mills have chemical recovery furnaces or boilers with direct contact evaporators which may require BLOX systems to treat the black liquor to reduce the level of residual sulfides. BLOX is an exothermic process, and thus results in a decrease in the heating value of the oxidized black liquor.